Infection in rabbits injected with HIV-1 has been detected by seroconversion, detection of virus in cells and organs using PCR and in situ hybridization, and by isolation of virus from PBMC. While the fact that rabbits can be infected with HIV is promising, the slow course of infection, difficulty in isolation of virus and the absence of consistent disease signs in infected rabbits have limited utility of this model for AIDS. Information gained in recent efforts aim to improve the model suggest that infection in the rabbit may be enhanced in several ways. Adaptation of HIV to grow more efficiently in rabbits has been attempted by long term in vivo passage; injection of as little as 1 ml of blood from animals receiving splenocytes from a rabbit infected with HIV-1 and held for 2.3 years has resulted in lymphopenia, CD4 T cell depletion and for 2 of 12 rabbits, death. Initial results from structural analyses of HIV-1 amplified from a spleen cell recipient shows nucleotide sequence identity of 60-85% to the original input isolate, HIVLai. Compared to HIV-1, HTLV- I infects rabbit cells with high efficiency. Comparison of Northern Blots from HIV-1 and HTLV-I infected rabbit cells indicated that HIV-1 transcripts are relatively unstable, while those of HTLV-1 are as stable as endogenous message. The efficacy of HTLV-1 infection in rabbit cells has prompted preparation of CAT constructs utilizing the 5' or 3' LTRS from HIV-1 or HTLV-I to provide the promoter or 3' processing signals. These are being used to compare LTR-mediated gene expression in rabbit and human cells. Data from these expression studies will be used to guide the construction of HIV-1 clones containing alternative LTR sequences.